According to a conventional particle beam irradiation apparatus for performing scanning irradiation, as disclosed in Patent Document 1, set current for a scanning electromagnet as scanning means is varied with time lapse to scan a charged particle beam. The value of the set current of the scanning electromagnet can be calculated according to a theoretical formula based on a specification of the scanning electromagnet, a specification of a power source for the scanning electromagnet and a specification of an irradiation beam (irradiation energy, incident beam position, etc.). However, the set current value for the scanning electromagnet calculated according to the theoretical formula is a theoretical value based on the premise that the specification of the scanning electromagnet, the specification of the scanning power source and the specification of the irradiation beam are never changed. Actually, the set current value is changed due to various factors, so that the irradiation position may be displaced and thus erroneous irradiation may occur.
For example, the scanning electromagnet is generally a bipolar electromagnet. Therefore, the beam irradiation position is potentially displaced from an estimated position due to residual magnetic field caused by hysteresis of the electromagnet although no current is supplied to the electromagnet. Furthermore, the beam irradiation position is also potentially displaced due to some secular variation of equipment although beams are irradiated under the same condition. Therefore, there has been proposed a method in which plural appropriate irradiation conditions (irradiation energy, etc.) are set to perform test irradiation under a state that no patient exists before a medical treatment is conducted or the like, a conversion table representing the relation of beam position data (xa, ya) detected on a beam position monitor and set current values (Ia, Ib) of the scanning electromagnet are stored in a storage device in advance and then a set current value of the scanning electromagnet is calculated by using the conversion table when irradiation for a treatment is performed (Patent Document 1).